1. Field of the Invention
This invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention pertains to primary lithium/silver vanadium oxide (Li/SVO) cells with improved voltage delay characteristics and lithium-containing secondary cells with improved cycling efficiency. According to the present invention, the activating electrolyte for these electrochemical systems is provided with a gaseous additive for the purpose of beneficially modifying the lithium passivation film. Additionally, electrolyte preparation is carried out in an atmosphere of the gaseous material. This ensures that the intended concentration of the additive in the electrolyte is maintained. Furthermore, lithium-containing primary and secondary cells are activated by filling the additive-containing electrolyte into the casing in the same gaseous additive atmosphere. These procedures significantly improve the voltage delay characteristics of a Li/SVO cell and the cycling efficiency characteristics of a lithium-containing secondary cell in comparison to prior art cells of similar chemistries because very little, if any, of the gaseous additive evaporates from the electrolyte. Essentially, the intended concentration of addition is maintained in the electrolyte to improve the voltage delay and cycling characteristics of the respective cells.
2. Prior Art
Many implantable medical devices use a power source which consists, in part, of a primary lithium electrochemical cell. In most instances, the primary cell is of a lithium/silver vanadium oxide couple. This chemistry has proven to be a reliable and dependable power source which can, if needed, deliver a pulse current discharge such as is required by implantable cardiac defibrillators.
Recently, the pulse current discharge of Li/SVO electrochemical cells has been improved by the provision of various types of additives to the electrochemical chemistry. These additives help alleviate, and in some cases eliminate, the voltage delay phenomenon present during various stages of cell discharge. Voltage delay manifests itself as a temporary decrease in cell voltage during application of a pulse current. It is generally attributed to an increase in the resistance of the lithium passivation layer on the surface of the anode electrode which impedes the flow of lithium ions from the anode into the electrolyte during pulse current discharge and results in a temporarily lower voltage exhibited by the cell. In some instances, the life of the implantable device may be severely reduced.
Modification of the anode passivation layer by the inclusion of an additive in the electrochemical chemistry results in the formation of an ionically conductive protective film thereon. This protective film greatly reduces, or even eliminates, the voltage delay phenomenon, and is primarily accomplished by the formation of a salt of one of a number of additives classified as nitrites, nitrates, carbonates, dicarbonates, phosphonates, phosphates, sulfates, and sulfites on the surface of the lithium metal. The resulting salt is more conductive than lithium oxide which may form on the anode in the absence of the electrolyte additive. In fact, it is believed that the lithium additive salt or lithium salt of the additive reduction product on the surface of the anode provides for the existence of charge delocalization due to resonance equilibration at the anode surface. This equilibration allows lithium ions to travel easily from one molecule to the other via a lithium ion exchange mechanism. As a result, beneficial ionic conductance is realized.
Several methods of beneficially modifying the lithium passivation film have been shown to be successful. These include exposing freshly scraped lithium metal to a gaseous form of the additive prior to inclusion of the active material into the cell assembly, providing a solid form of the additive, when appropriate, in the electrolyte or dissolving a gaseous form of the additive into the electrolyte. These and other methods may be employed alone or in combination with each other.
One method in particular is preferred because of its lower cost. This involves saturating the electrolyte with a gaseous form of the additive. For example, when the additive is a carbonate, carbon dioxide is easily saturated into the electrolyte. The modified passivation layer on the surface of the anode electrode forms in-situ when and immediately after the cell is filled with the electrolyte.
Although process economics favor the use of an electrolyte that is saturated with gaseous carbon dioxide for alleviating the voltage delay phenomenon, application of this method to defibrillator batteries has yet to occur. Previously, lithium cells activated with such an electrolyte exhibited inconsistent improvements in alleviating the voltage delay phenomenon. Although lithium cells containing a nonaqueous electrolyte saturated with carbon dioxide do not exhibit worsened voltage delay than historically observed, improvements are not consistently observed from one cell to the next and from batch to batch. Such unpredictability is not acceptable for cells intended to power implantable medical devices, such as cardiac defibrillators.
Concentration measurements suggest that the inconsistent effects are due to the processes used to make and store the electrolyte and to fill the cell. Specifically, it has been determined that gaseous carbon dioxide degasses from the electrolyte during storage and during vacuum filling of the cell. The degassing effect becomes more pronounced over time and results in a significant difference in concentration levels from lot to lot, and even in cells activated with the same electrolyte lot. In essence, cells activated with the same electrolyte lot contain varying amounts of dissolved carbon dioxide, and this results in inconsistent alleviation of the voltage delay phenomenon. This also results in variations in the cycling efficiency from one secondary cell to the next.
The reason for this in a primary cell is that during the application of a series of pulse currents, the passivation layer on the anode electrode is disrupted, resulting in the loss of some lithium carbonate species from the surface film. After the pulsing is completed, the passivation layer reforms using an additional amount of dissolved carbon dioxide within the electrolyte to re-form the lithium carbonate surface species via reaction with freshly exposed lithium metal. When all of the dissolved carbon dioxide is reacted, which occurs prematurely in cells filled towards the end of an electrolyte lot and from which appreciable amounts of dissolved carbon dioxide have degassed, the lithium carbonate species no longer forms within the passivation layer. The traditional passivation layer is then present and the alleviation or elimination of the voltage delay phenomenon is no longer possible. A similar phenomena occurs in the cycling of a lithium-containing secondary cell.
Accordingly, there is needed a method for introducing a gaseous additive into the electrolyte intended for activating a lithium electrochemical cell, of either the primary or the secondary types, so that the intended concentration of additive is maintained throughout electrolyte preparation and filling of the casing. Maintaining the intended additive concentration in the electrolyte ensures alleviation, and in some cases elimination, of voltage delay throughout the useful life of the cell.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.